176 MAINE STATE COLLEGE 



leaves and tubers of the host plant, running between the cells. 

 The mycelium does not produce lateral branches (haustoria) for 

 obtaining nourishment from the cells of the host plant. 



Conidia. — The mycelium sends, during the summer months, slen- 

 der branches {conidiophores) through the storaata or breathing 

 p,ores of the under side of the leaves. The conidiophores are at 

 first simple, but finally produce a few lateral, irregularly placed 

 branches. They are attenuated at the apex and enlarged at the 

 point wheie the conidia are located. The conidia or summer 

 spores are borne on these branches. They are eliptical or oval 

 bodies 27 to 29 micromillimetres in length, with a terminal projec- 

 tion at the base. The conidia are at first single at the end of the 

 branches but finally become lateral by the outgrowth of the tip of 

 the branch. Several spores may be produced in succeasion. These 

 conidia may under proper conditions develope germ threads 

 directly, which penetrate the breathing pores of the host, producing 

 a new plant. More conimonly the conidia produce zoospores. 

 The contents of the conidia breaks up into several masses, which 

 are finally liberated by a rupture of the wall at the small end. 

 These minute, jelly-like (protoplasmic) bodies have the power of 

 moving about with a jerky motion, like small animals, by means of 

 two slender cilia or hairs attached to one side of the spore. On 

 account of this power of motion they are called zoospores. The 

 zoospores soon come to rest, lose the cilia, assume a rounded form 

 and develope a thin membanous wall. Under favorable circum- 

 stances they send forth germ threads which penetrate the breath- 

 ing pores of the potato leaves and become new plants. 



Zoospores. — These are egg spores, resting spor«s, or winter 

 spores as they are variously called. They are supposed to develop 

 within the stem, leaves and tubers of the host plant and perpetuate 

 the species. Though they are well known in related species, there 

 is a difference of opinion regarding their occurrence in the Potato 

 Rot. 



The species has wonderful power of perpetuating itself. Some 

 believe as winter spores in the stem, leaves and tubers; others 

 regard the mycelium perennial in the stored tubers, and the seed 

 the means of infection ; others, (the writer belongs to that class,) 

 do not believe the preservation of the mycelium in seed potatoes 

 adequate to account for the continuance and spread of the disease. 

 The writer believes there is some source of infection left behind 



